1. Field of the Invention
The present invention relates to a phototaking optical system and an optical element and, more particularly, to a phototaking optical system and an optical element which are suitably used for an optical device such as a camera.
2. Related Background Art
As a conventional device having a reflection surface as part of an optical system, for example, a telescope like the one shown in FIG. 1 is available. The telescope shown in FIG. 1 is of a type called a Cassegrain reflecting telescope. This telescope is constituted by a concave mirror 51, a convex mirror 52, and an eyepiece 55. Parallel light 54 from an object at an infinite distance is reflected as a convergent light beam by the concave mirror 51 toward the object side. The convergent light beam is reflected by the convex mirror 52, placed on the object side of the concave mirror 51, toward the observer side to form an object image on an intermediate imaging plane 53. Thereafter, the object image is observed by using the eyepiece 55 placed behind the intermediate imaging plane 53.
As described above, in the Cassegrain reflecting telescope, the optical path of the telephoto lens system constituted by the refracting lenses and having a long total lens length is folded by using the two reflecting mirrors so as to decrease the overall length of the optical system. However, in the Cassegrain reflecting telescope, since a real image is formed, and the eyepiece is placed behind the image, a reduction in size in the longitudinal direction is limited.
In contrast to this, a reflecting optical system designed to attain a reduction in size by integrating reflection and refraction surfaces is also known.
FIG. 2 is a schematic view showing the main part of an observation optical system disclosed in U.S. Pat. No. 4,775,217. This observation optical system is an optical system which is used to observe an outdoor scene, together with a display image displayed on an information display unit, in an overlapping state.
In this observation optical system, a display light beam 65 emerging from a display image on an information display unit 61 is reflected by a surface 62 toward the object side and incident on a concave half mirror surface 63. After being reflected by the half mirror surface 63, the display light beam 65 is converted into an almost parallel light beam owing to the refracting power of the concave surface 63. The parallel light beam is refracted/transmitted through the surface 62 to form an enlarged virtual image of the display image at an infinite distance on the object side. At the same time, the display light beam 65 is incident on a pupil 64 of the observer to allow him/her to recognize the display image.
Meanwhile, an object light beam 66 from an object is incident on a surface 67 almost parallel to the reflection surface 62. The light beam is refracted and reaches the concave half mirror surface 63. A semi-transparent film is deposited on the concave surface 63. The object light beam 66 is partly transmitted through the concave surface 63 and refracted/transmitted through the surface 62. The light beam is then incident on the pupil 64 of the observer. With this operation, the observer can visually recognize the display image in the outdoor scene in an overlapping state.
FIG. 3 is a schematic view showing the main part of an observation optical system disclosed in Japanese Laid-Open Patent Application No. 2-297516. This observation optical system is also an optical system which is used to observe an outdoor scene, together with a display image displayed on an information display unit, in an overlapping state.
In this observation optical system, a display light beam 74 emerging from an information display unit 70 is transmitted through a plane 77 as a part of an optical member Pa and enters the optical member Pa to be incident on a parabolic reflection surface 71. The display light beam 74 is reflected by the reflection surface 71 to become a convergent light beam, thereby forming an image on a focal plane 76.
At this time, the display light beam 74 reflected by the parabolic reflection surface 71 has reached the focal plane 76 after being totally reflected by the two parallel planes constituting the optical member Pa, i.e., the plane 77 and a plane 78. With this arrangement, a low-profile optical system is attained.
The display light beam 74 emerging as divergent light from the focal plane 76 is totally reflected by the planes 77 and 78 and incident on a half mirror 72 constituted by a parabolic surface. The light beam is reflected by the half mirror 72. At the same time, this light beam forms an enlarged virtual image of the display image owing to the refracting power of the half mirror 72 and becomes an almost parallel light beam. The light beam is transmitted through the plane 77 and incident on a pupil 73 of the observer, thereby allowing the observer to recognize the display image.
Meanwhile, an external object light beam 75 is transmitted through a surface 78b as a part of an optical member Pb and transmitted through the half mirror 72. The light beam is then transmitted through the plane 77 and incident on the pupil 73 of the observer. The observer then visually recognizes the display image in the outdoor scene in an overlapping state.
The observation optical system shown in FIG. 2 is constituted by the flat refraction surface and the concave half mirror surface to attain a reduction in size. However, the surface 62 from which light beams from the information display unit and the outdoor scene emerge is not designed to perform aberration correction because the surface 62 is used as a total reflection surface for a light beam from the information display unit 61.
The observation optical system shown in FIG. 3 is designed to attain a reduction in size by using the flat refraction surface, the parabolic reflection surface, and the half mirror constituted by the parabolic surface. However, the incident and exit surfaces for an object light beam from an outdoor scene are not designed to perform aberration correction because the extended surfaces of the incident and exit surfaces are used as total reflection surfaces for guiding a light beam from the information display unit 70.